From the viewpoint of processability, handlability, and production cost, etc., a plastic optical fiber is more favorable than a glass-based optical fiber, and thus it is suitably used for short-distance optical communication transmission, photoelectric sensor, and light guide, etc.
Recently, as automotive interior wiring for information communication, a plastic optical fiber cord containing a plastic optical fiber coated with a thermoplastic resin such as nylon (polyamide) is used.
When used for house wiring or automotive communication wiring, the plastic optical fiber cord is often installed in a small area with high temperature and high humidity condition in a bending manner, thus properties such as heat resistance, wet heat resistance, folding resistance, and bending loss resistance are required therefor.
In particular, since the temperature around the wiring in the roof or engine room of an automobile is high, i.e., about 100° C., a plastic optical fiber cord satisfying long-term resistance to heat as such high temperature of 100 to 105° C. has been waited for.
The plastic optical fiber cord is generally attached with a connector on its end and used. Since a bare (unjacketed) plastic optical fiber can easily receive a scratch when the coating layer on the surface of a plastic optical fiber cord is stripped, a method of attaching and fixing a connector part while maintaining the coating layer is employed. When a coating layer is attached and fixed on a connector part, it is necessary to increase adhesion between the bare (unjacketed) plastic optical fiber and the coating layer as well as heat resistance to maintain the connectivity strength between the connector and the plastic optical fiber cord.
The plastic optical fiber cord is constituted of two kinds of polymers, i.e., a core and a cladding. Conventionally, a polymer such as polymethyl methacrylate (herein below, it may be abbreviated as PMMA) having excellent transparency and good weather resistance has been used for the core. Meanwhile, to store light inside the core, it is required for a cladding to have lower refractive index than the core, and a fluorine-containing polymer is widely used for a cladding.
There is conventional technology for improving heat resistance of a plastic optical fiber cord by using PMMA as a core.
A plastic optical fiber cord using an α-fluoroacrylate copolymer as a cladding material, which is non-crystalline and has high glass transition temperature, and using nylon 12, polypropylene or the like as a coating material is known. However, the cladding material based on α-fluoroacrylate copolymer is very expensive. Further, as the transparency of the cladding material itself is poor, initial transmission loss is also bad. Still further, as the interface adhesion with a core is poor, there is a problem that mechanical properties such as folding resistance are not good.
Further, a plastic optical fiber cable obtained by coating the outer periphery of an unjacketed plastic optical fiber with a coating layer comprising polyamide resin composition, wherein the first cladding comprises a polymer having 15 to 90% by weight of fluoroalkyl(meth)acrylate unit (A) and 10 to 85% by weight of other copolymerizable monomer unit (B) and the second cladding comprises a fluorine-containing olefin resin having a tetrafluoroethylene unit, is known. However, the fluoroalkyl(meth)acrylate copolymer, which is used as a cladding material of a plastic optical fiber, is highly expensive and has poor interface adhesion with a core, and therefore there is a problem that mechanical properties such as folding resistance are not good.
Further, a plastic optical fiber cable obtained by forming on the outer periphery of an unjacketed plastic optical fiber a coating layer comprising nylon 12, wherein the core comprises PMMA and the cladding comprises a terpolymer having 40 to 62 mol % of a vinylidene fluoride unit, 28 to 40 mol % of a tetrafluoroethylene unit, and 8 to 22 mol % of a hexafluoropropylene unit, is known. However, the plastic optical fiber wherein the cladding material comprises the terpolymer of vinylidene fluoride, tetrafluoroethylene, and hexafluoropropylene has a problem that it has poor long-term heat resistance and poor wet heat resistance.
Further, an optical fiber cable obtained by forming on the outer periphery of an optical fiber a coating layer comprising a thermoplastic resin, wherein the core comprises PMMA and the cladding comprises a terpolymer having 5 to 30% by weight of an ethylene unit, 40 to 75% by weight of a tetrafluoroethylene unit, and 15 to 50% by weight of a hexafluoropropylene unit, is known (Japanese Patent Application Laid-Open (JP-A) No. 2001-074944). However, the ethylene/tetrafluoroethylene/hexafluoropropylene copolymer used as a cladding material has a problem that it has poor heat resistance at temperature condition of 105° C. and it cannot withstand at all for the use at a temperature of 105° C.
It could therefore be helpful to provide a plastic optical fiber and a plastic optical fiber cord having such a level of heat resistance that they can withstand the use at an actual use temperature of 105° C.